Light emitting devices (herein after LEDs) have recently become widely used in many lighting applications, such as flashlights, traffic signals, automotive tail lights and display devices because the LEDs are known to provide a significantly higher power efficiency, smaller size and longer operating life compared to conventional incandescent lights.
Among the various packages for LEDs, the plastic leaded chip carrier (herein after PLCC) package for a surface mount device (herein after SMD) is one of the more popular packages for high power applications. For example, surface mount LEDs in a PLCC package may be used in automotive lighting and electronic signal lights. A typical SMD LED often comprises a lead frame over molded with a polymer material such as Polyphthalamide (PPA), Polyamide or Epoxy resin encapsulant like MG 97. Typically, an LED or die is disposed inside a reflective cup and attached to the lead frame using an adhesive and an electrical connection is established between the LED and lead frame by means of wire bonding. A transparent encapsulant, usually epoxy or silicone, is then injected into the cavity of the reflective cup so that the LED and wire is protected from the environment.
In many conventional SMD LED, a lens is utilized for shaping the light viewing angle and also to protect the gel type encapsulant within the package. Typically, a lens is attached to the top of the LED by means of an adhesive material. One drawback of this conventional LED design is that there is often poor adhesion between the lens and the body. Moreover, the adhesive material used to attach the lens to the LED may tend to degrade over time, especially given the repeated thermal cycling that high power LEDs tend to go through. In some cases, if the package is subjected to repeated thermal cycling, any differences or mismatches in thermal properties between the adhesive material, the LED die, and the lens material, may cause these materials to expand and contract differently, resulting in internal thermal stress. Such internal thermal stress may cause the adhesive material to degrade over time and the lens to detach from the package.
Another conventional SMD LED design may include a lens with a guided pin coupled to one or more guide holes in the package, where the guided pins and guide holes provide alignment for lens attachment as well as a mechanical interlocking means. Many times with this design, either an extra large package is required to accommodate the large lens with one or more guided pins, an adhesive is used in addition to the guide pins and guide holes to provide interlocking means, or both larger overall package and an adhesive are utilized. However, due to the large size of such a package design, the issues discussed above regarding adhesive degradation over time, and the complexity in its structure, such a system would not be reliable and may be susceptible to lens detachment over time.
In view of these concerns, there is a need for a compact package design for high power surface mount light emitting devices with enhanced attachment between the lens and the package and a method for making such a package.